1. Field of the Invention
This invention relates to a method for improving the heatproofness and chemical resistance of preflux to be used as for soldering metals. It further relates to a printed wiring board which, in the operation of mounting electronic parts in high density on the opposite surfaces of the printed wiring board and reflow soldering them to the surfaces, allows the surfaces thereof to be wetted thoroughly with the solder and enables the soldering inside through holes formed therein to be carried out with enhanced efficiency and to a method for the production of the printed wiring board.
2. Description of the Prior Art
The prefluxes which have been heretofore used for the purpose of rustproofing circuit parts of copper or copper alloy formed on a printed wiring board and intended to be soldered and retaining the circuit parts in a solderable state are broadly divided into two kinds, i.e. a resin type preflux intended to coat a printed wiring board wholly and an alkylimidazole type preflux intended to produce a chemical reaction selectively with the copper or copper alloy.
The former preflux is used by a method which comprises dissolving natural rosin, rosin ester, or rosin-modified maleic acid resin in an organic solvent, applying the resultant solution to a whole printed wiring board by spraying with a roll coater, immersion, or the combination thereof, and drying the applied layer of the solution thereby forming a film coating the printed wiring board.
The resin type preflux, owing to the use of the organic solvent, entails the disadvantage that the solvent evaporates into the ambient air and seriously jeopardizes environment and safety.
The alkylimidazole type preflux is soluble in water and excels in point of work environment and safety. It nevertheless entails the disadvantage that an alkylimidazole copper complex arising from the reaction of the preflux with the copper or copper alloy of the printed wiring board, on exposure to an elevated temperature, is decomposed and degenerated by the catalytic action of oxygen in the air and copper and consequently suffered to adhere fast to the circuit of copper or copper alloy, impede the action of a postflux, and impair the solderability of the circuit.
In recent years, the alkylimidazole type preflux has come to find growing utility on account of work environment and safety.
The printed wiring board is produced by a method which comprises applying the alkylimidazole type preflux to the printed wiring board and then reflow soldering electronic parts to the surfaces of the printed wiring board thereby effecting desired mounting of the electronic parts on the printed wiring board.
The reflow soldering techniques which have been developed to date to the level of commercialization and actually adopted for mounting electronic parts on the surfaces of a printed wiring board include air reflow, infrared reflow, near-infrared reflow, reflow of the type combining air reflow and infrared reflow, reflow performed in the atmosphere of nitrogen, and vapor reflow for performing soldering in the vapor phase of perfluorocarbon, for example.
When the alkylimidazole type preflux is applied to a surface as described above and then the reflow soldering is performed on the coated surface, the preflux which is consequently exposed to an elevated temperature poses the problem of heatproofness and chemical resistance. Thus, the surface of copper and copper alloy is invaded by a halogenide contained in a cream solder which is used as a solder and the moisture evaporating from within a printed wiring and the evaporating moisture goes to decomposing the perfluorocarbon, though only to a slight extent, with liberation of hydrofluoric acid and inevitably inducing corrosion of the surface of copper, with the result that the susceptibility of the surface to wetting with the solder during the process of reflow soldering, the efficiency of the soldering work performed subsequently to the step of reflowing in through holes formed in the printed wiring board, and the susceptibility of the through holes to wetting with the solder are degraded. Such is the true state of affairs.
The inventors, in the light of the problem mentioned above, continued a study in search of a metal soldering grade preflux endowed with improved heatproofness and chemical resistance, a printed wiring board susceptible to perfect wetting with solder even after two to three rounds of reflow soldering and capable of undergoing soldering with enhanced efficiency, and a method for the production of this printed wiring board. Consequently, they have found that their object is accomplished by performing an oxidizing treatment on an alkylbenzimidazole or a derivative thereof represented by the following general formula (1): ##STR1## (wherein R.sub.1 stands for H or an alkyl group of three to 17 carbon atoms, R.sub.2 for H or an alkyl group of one to six carbon atoms, and n for an integer in the range between 0 and 3) or the following general formula (2): ##STR2## (wherein R.sub.3 for H or an alkyl group of one to six carbon atoms, R.sub.4 for H or an alkyl group of one to six carbon atoms, R.sub.5 for an alkyl group of zero to seven carbon atoms, and n for an integer in the range between 0 and 3) and produced by a chemical reaction on the soldered part of copper or copper alloy of a printed wiring board.
They have also perfected a method for the production of a printed wiring board which is enabled to acquire improved heatproofness and chemical resistance and manifest perfect and efficient solderability and susceptibility to wetting with the solder even after two to three rounds of reflowing.